I only have been staying on top of alk but do have CA and mg tests..I was just told my zoa's and mushroom don't require me to monitor CA and mg so I haven't been.
To me it looks like a small plate coral next to the zoas. My guess is they will battle and the plate coral will lose. Plates only have one mouth so I don't think they can be fragged. My understanding is that once they start to go
it's over. I had a beautiful long tentacle plate that was about 7" in diameter when inflated. I woke up one morning and found one of my green hairy mushrooms pressed up against the side of it. I moved it off it, but it was gone within 2 weeks.
Cut the plate coral off from the base, set it in the sand and will grow super fast. I had the same exact sort of colony and the plate was just the size of a dime. Thought it was bad thing but now this is what the plate looks like... Its about 2" wide maybe a bit more.